grub: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On CentOS possible default full paths are:

/sbin/grub - from package: grub

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/sbin/grub - from package: grub-legacy

You may install the required package with command apt-get install {package}

Error in other languages:
grub: bevel nie gevind nie
grub: comandă negăsită
grub: comando não encontrado
grub: commande introuvable
grub: command not found
grub: käsku ei ole
grub: không tìm thấy lệnh
grub: komanda nerasta
grub: Kommando nicht gefunden.
grub: kommandot finns inte
grub: komut yok
grub: nie znaleziono polecenia
grub: níor aimsíodh an t-ordú
grub: no se encontró la orden
grub: no s'ha trobat l'ordre
grub: opdracht niet gevonden
grub: parancs nem található
grub: perintah tidak ditemukan
grub: príkaz nenájdený
grub: příkaz nenalezen
grub: команда не найдена
grub: командата не е открита
grub: 命令找不到
grub: コマンドが見つかりません